PhD in chemical engineering from University of California Berkeley. Registered Professional Engineer. Professor at University of Colorado, University of Cincinnati, and National University of Singapore. Expertise in membrane science
Abstract Membrane distillation (MD) can concentrate non-volatile solutes or remove volatiles and ... more Abstract Membrane distillation (MD) can concentrate non-volatile solutes or remove volatiles and dissolved gases from an aqueous feed. A microporous hydrophobic membrane provides a barrier between the hot feed and cold distillate. Although MD can operate at ambient pressure and moderate temperatures, use waste heat, and treat wastewater via an MD-bioreactor, it has problems such as temperature polarization, liquid weeping to the distillate side, and membrane fouling. Prior studies speculated that fouling can add a heat- or mass-transfer resistance, or cause a vapor-pressure reduction owing to the Kelvin effect, but did not isolate these effects. This study confirms that the vapor-pressure depression owing to the concave interface in the small pores of the fouling layer is a dominant cause of the 25–63% flux reduction observed for humic-acid fouling on PTFE and PVDF membranes. This study underscores the importance of selecting MD membranes based on their pore-size distribution rather than just their nominal diameter in order to maximize the contribution of Knudsen diffusion. It suggests the development of dual-layer membranes having a thin hydrophilic layer with relatively large pores overlying a hydrophobic layer with a typical MD membrane structure in order to mitigate the vapor-pressure reduction owing to membrane fouling.
The method of activation energy asymptotics has been applied to analyze the zeroth-order dynamics... more The method of activation energy asymptotics has been applied to analyze the zeroth-order dynamics of a planar combustion wave traveling through a porous medium in a direction opposite to that of the forced oxygen flux. Such a process finds practical application in the “reverse combustion” of coal seams in-situ. This two-phase oxygen-limited combustion process assumes an infinite effective Lewis number. The adiabatic front temperature is the eigenvalue for the basic-state problem, and is related to the front velocity by the integral energy balance.
Rayleigh convection of water in the permafrost active layer is modeled to explain regularity, siz... more Rayleigh convection of water in the permafrost active layer is modeled to explain regularity, size, shape for some types of sorted patterned ground. Uneven melting of the ice-water interface results in an undulatory ice front of regularly spaced peaks and troughs that retreats downward throughout the summer thaw. This paper presents an original solution to the Rayleigh convection problem with a moving lower boundary. The predicted convection cell width to active layer depth ratio is corroborated with field excavations of sorted patterned ground, and additionally, the significance of Rayleigh convection in engineering design is discussed.
Taking a sabbatical or academic leave is becoming less popular owing to the problems associated w... more Taking a sabbatical or academic leave is becoming less popular owing to the problems associated with dual income families and the need to keep research programs running. This paper advances strong arguments for fitting well-timed leaves into one’s long-range academic plans. The author has taken four sabbaticals and four academic leaves during the 37 years of his full-time academic career and has had four extended visiting faculty appointments since his retirement. His experience has included appointments at one U.S. and nine foreign universities, the National Science Foundation and two multinational companies. This paper will discuss the advantages of taking sabbatical and academic leaves to one’s teaching, research, cultural, and personal development. Examples will be given on how a leave experience made an irrevocably positive difference in the author’s teaching. Information also will be given on how to plan for a leave. In particular, this paper will discuss possible sources of f...
Sorted patterned ground refers to polygons, nets, or stripes defined by rocky borders which are t... more Sorted patterned ground refers to polygons, nets, or stripes defined by rocky borders which are the result of sorting in soil subjected to frost action. This paper presents a model in which convection cells, driven by unstable density stratification in the aqueous phase, result in uneven melting of the underlying ice front during thawing. The resulting undulatory ice front with regularly spaced peaks and troughs provides the pattern which results in the regularity observed for certain types of patterned ground. In addition, the model predicts the width to depth-of-sorting ratio for both polygons and stripes, and explains the characteristic hexagonal shape of sorted polygons, the transition from sorted polygons to sorted stripes on sloped terrain, and the formation of sorted polygons under water. The predicted ratio of width to depth-of-sorting of 3.81 for sorted polygons is compared with the ratio of width to depth-of-sorting of 3.57 found from a linear regression analysis of 18 fie...
The various processes by which polymeric membranes are formed all involve the solidification of a... more The various processes by which polymeric membranes are formed all involve the solidification of an initially homogenous liquid phase. In order to validate a predictive model for these formation processed, it is necessary to make real-time noninvasive measurements of important process parameters. Although techniques have been described recently for the acquisition of mass and temperature data, no procedures have been reported for the measurement of instantaneous film thickness even though this is a particularly critical process variable. In response to this need, the authors have developed a novel laser interferometry technique which permits changes as small as 0.2 microns in film thickness to be measured. Utilizing this technique, real-time film thickness data for the cellulose acetate-acetone-water system have been obtained and compared with the dry-cast process model developed by the authors` group. It has been found that the experimental results and the model predictions are in good agreement. The theoretical basis for interference measurements, the experimental considerations in the utilization of a laser light source and the general applicability of the experimental technique also will be discussed.
ABSTRACT Better detection and control of fouling in liquid-separation processes is essential if m... more ABSTRACT Better detection and control of fouling in liquid-separation processes is essential if membranes are to find increased use in a variety of industrial applications. Ultrasonic time-domain reflectometry (UTDR) is an in situ, noninvasive real-time technique that has been successfully utilized to quantify fouling and cleaning of flat sheet membranes. This study describes the extension of UTDR for the measurement of fouling and cleaning in commercial membrane modules employing spiral-wound elements. Experiments were conducted using a high-pressure separation system incorporating a commercial spiral-wound module. Calcium sulfate was utilized as the primary scalant in concentrations ranging from 1.2-1.8 g/L; ferric hydroxide colloidal fouling also occurred during the tests. A multi-cycle protocol was adopted whereby each cycle consisted of three stages: pure water equilibration, fouling and cleaning. Permeation rate as well as acoustic amplitude and arrival time measurements were made at regular intervals during three complete cycles on two modules. A new signal analysis protocol was developed such that systematic changes in the entire acoustic spectrum as a function of module operation time could be represented in terms of shift factors. In addition, gravimetric, microscopic and spectroscopic measurements were made at the end of the experiments so that the extent of the membrane fouling and cleaning could independently assessed. Results indicate that the overall decline in permeation rate is reasonably correlated with change in both the amplitude and arrival time shift factors. Overall, this work confirms the effectiveness of the UTDR technique and provides a sound basis for continued development with an ultimate goal of providing on-line sensors for the timely detection of fouling and cleaning of commercial liquid-separation membrane systems.
ABSTRACT In this study an on-line feed fouling monitor (FFM) combined with a salt-tracer-response... more ABSTRACT In this study an on-line feed fouling monitor (FFM) combined with a salt-tracer-response technique (STRT) was used to predict reverse osmosis (RO) fouling under constant flux filtration. The FFM was used to capture foulant loads using a small ‘collection’ ultrafiltration (UF) membrane at the same crossflow hydrodynamics as in the RO experiments. A UF membrane was used in the FFM to decrease the monitoring time and improve the accuracy because it is more responsive to the fouling resistance than an RO membrane. Since the deposits captured by the FFM are potential RO foulants, the resulting information can be used to predict the transmembrane pressure (TMP) rise for the RO membrane. The STRT was used to measure the development of concentration polarization that is important in estimating the cake-enhanced osmotic pressure (CEOP) contribution. A model was developed that includes both the cake resistance and the CEOP effect due to cake formation and was used to predict RO fouling trends. Model foulants were humic acid and colloidal silica. The major focus was on organic fouling by humic acid (20 mg/l) in 2000 mg/l sodium chloride (NaCl) as the ionic background for the RO and FFM fouling experiments. The RO and FFM fouling experiments were conducted at different constant fluxes using the same feed solutions and at the same crossflow velocity (0.1 m/s). The results indicated that higher fluxes cause an increased fouling rate for both RO and the FFM for both types of solute. The CEOP effect, measured by the salt-tracer-response in the RO experiments, was also strongly enhanced by the flux. The model was validated by plotting the predicted RO transmembrane pressure (TMP) as a function of time for different fluxes based on the resistivity from the FFM and the CP obtained from the STRT. For both organic foulants and colloidal silica the results show that the combination of the FFM and salt-tracer-response STRT is a promising method to provide a good estimate of the RO fouling trends. It also underscores the contribution of CEOP to the increase in TMP during RO fouling of saline feeds.
ABSTRACT Ultrasonic Time Domain Reflectometry (UTDR) is used to monitor the deposition and physic... more ABSTRACT Ultrasonic Time Domain Reflectometry (UTDR) is used to monitor the deposition and physicochemical nature of colloidal silica fouling on a polyethersulfone ultrafiltration membrane under crossflow and constant flux conditions. The fouling can be characterized by four stages based on the rate of increase of the transmembrane pressure (TMP): (1) an initial rapid increasing rate due to concentration polarization; (2) a slow constant rate; (3) a nonconstant increasing rate associated with metastability of the colloidal silica; and (4) a constant rapid rate associated with Deleted ‘progressive compression of’. the foulant layer having a constant thickness. The destabilization of the colloidal particles in the foulant layer during stage 3 is associated with an increase in the UTDR peak amplitude. During stage 4 the foulant layer thickness reaches a plateau value that decreases with increasing crossflow velocity. The rheological behavior of the foulant layer is described by the Bingham plastic model for which the yield stress can be obtained from the UTDR data. When membrane cleaning is done by switching from a fouling to a non-fouling feed solution, marked decreases in both the foulant layer thickness and TMP are observed. However, a residual tightly bound foulant layer remains on the membrane whose thickness, determined by UTDR, corroborates well with off-line scanning electron microscopy analysis.
ABSTRACT In this paper RO colloidal fouling was predicted using an on-line Feed Fouling Monitor (... more ABSTRACT In this paper RO colloidal fouling was predicted using an on-line Feed Fouling Monitor (FFM) combined with a monitor based on ultrasonic time domain reflectometry (UTDR) under constant flux filtration. The FFM incorporated the relevant cross-flow hydrodynamics and detected the development of foulant load and resistance from a continuous sample passing over a small UF membrane. A UF membrane was used in the FFM, rather than a RO membrane, because it was more sensitive to fouling resistance and this decreased monitoring time and increased accuracy. The UTDR was coupled to a RO membrane cell to monitor the rate of cake thickness increase, which is the required information for the estimation of cake-enhanced osmotic pressure (CEOP). A model was developed to predict fouling in RO using data from the two monitors and combined both the cake resistance and the CEOP effect due to cake formation. RO and FFM fouling experiments were performed at different constant fluxes using the same feed solutions (200 mg/l SiO2, 2000 mg/l NaCl) and the same cross-flow velocity (0.1 m/s). The results showed that higher fluxes caused an increased fouling rate for both RO and the FFM. The foulant layer thickness measured by UTDR in the RO experiments increased faster at higher applied fluxes and the estimated CEOP effect was also strongly influenced by the flux. The results show that the model, combined with the FFM and UTDR measurements, can provide a good estimation of the RO fouling profile over a range of applied fluxes. This study also underscores the importance of CEOP in contributing to the increase in transmembrane pressure due to colloidal fouling in RO of saline feeds.
ABSTRACT Detecting biofouling non-invasively in real-time has been a long-standing challenge in m... more ABSTRACT Detecting biofouling non-invasively in real-time has been a long-standing challenge in membrane processes. Ultrasonic Time Domain Reflectometry (UTDR) offers the potential to address this challenge when used in a ‘canary cell’ that samples a slip stream from the feed to the membrane process. However, adapting UTDR to detect biofouling is problematic owing to the small difference in acoustic properties between a biofouling layer and water or a membrane. In this study UTDR has been adapted to detect biofouling by periodic dosing of colloidal silica as an ‘acoustic enhancer’. This novel UTDR technique was applied to detect biofouling on flat sheet polyethersulfone ultrafiltration (UF) and thin film composite polyamide reverse osmosis (RO) membranes in a canary cell flow configuration. The UTDR response correlates with the transmembrane pressure and off-line measurements of the biofilm thickness via confocal laser scanning microscopy, the bacterial count, and extra-polymeric cellular substances (EPS). The acoustic enhancer was shown to have no effect on the viability of the bacteria. Hence, UTDR can be used for the noninvasive real-time detection of biofouling in high pressure membrane processes such as RO by periodic dosing of an enhancer such as colloidal silica in a canary cell.
Abstract Membrane distillation (MD) can concentrate non-volatile solutes or remove volatiles and ... more Abstract Membrane distillation (MD) can concentrate non-volatile solutes or remove volatiles and dissolved gases from an aqueous feed. A microporous hydrophobic membrane provides a barrier between the hot feed and cold distillate. Although MD can operate at ambient pressure and moderate temperatures, use waste heat, and treat wastewater via an MD-bioreactor, it has problems such as temperature polarization, liquid weeping to the distillate side, and membrane fouling. Prior studies speculated that fouling can add a heat- or mass-transfer resistance, or cause a vapor-pressure reduction owing to the Kelvin effect, but did not isolate these effects. This study confirms that the vapor-pressure depression owing to the concave interface in the small pores of the fouling layer is a dominant cause of the 25–63% flux reduction observed for humic-acid fouling on PTFE and PVDF membranes. This study underscores the importance of selecting MD membranes based on their pore-size distribution rather than just their nominal diameter in order to maximize the contribution of Knudsen diffusion. It suggests the development of dual-layer membranes having a thin hydrophilic layer with relatively large pores overlying a hydrophobic layer with a typical MD membrane structure in order to mitigate the vapor-pressure reduction owing to membrane fouling.
The method of activation energy asymptotics has been applied to analyze the zeroth-order dynamics... more The method of activation energy asymptotics has been applied to analyze the zeroth-order dynamics of a planar combustion wave traveling through a porous medium in a direction opposite to that of the forced oxygen flux. Such a process finds practical application in the “reverse combustion” of coal seams in-situ. This two-phase oxygen-limited combustion process assumes an infinite effective Lewis number. The adiabatic front temperature is the eigenvalue for the basic-state problem, and is related to the front velocity by the integral energy balance.
Rayleigh convection of water in the permafrost active layer is modeled to explain regularity, siz... more Rayleigh convection of water in the permafrost active layer is modeled to explain regularity, size, shape for some types of sorted patterned ground. Uneven melting of the ice-water interface results in an undulatory ice front of regularly spaced peaks and troughs that retreats downward throughout the summer thaw. This paper presents an original solution to the Rayleigh convection problem with a moving lower boundary. The predicted convection cell width to active layer depth ratio is corroborated with field excavations of sorted patterned ground, and additionally, the significance of Rayleigh convection in engineering design is discussed.
Taking a sabbatical or academic leave is becoming less popular owing to the problems associated w... more Taking a sabbatical or academic leave is becoming less popular owing to the problems associated with dual income families and the need to keep research programs running. This paper advances strong arguments for fitting well-timed leaves into one’s long-range academic plans. The author has taken four sabbaticals and four academic leaves during the 37 years of his full-time academic career and has had four extended visiting faculty appointments since his retirement. His experience has included appointments at one U.S. and nine foreign universities, the National Science Foundation and two multinational companies. This paper will discuss the advantages of taking sabbatical and academic leaves to one’s teaching, research, cultural, and personal development. Examples will be given on how a leave experience made an irrevocably positive difference in the author’s teaching. Information also will be given on how to plan for a leave. In particular, this paper will discuss possible sources of f...
Sorted patterned ground refers to polygons, nets, or stripes defined by rocky borders which are t... more Sorted patterned ground refers to polygons, nets, or stripes defined by rocky borders which are the result of sorting in soil subjected to frost action. This paper presents a model in which convection cells, driven by unstable density stratification in the aqueous phase, result in uneven melting of the underlying ice front during thawing. The resulting undulatory ice front with regularly spaced peaks and troughs provides the pattern which results in the regularity observed for certain types of patterned ground. In addition, the model predicts the width to depth-of-sorting ratio for both polygons and stripes, and explains the characteristic hexagonal shape of sorted polygons, the transition from sorted polygons to sorted stripes on sloped terrain, and the formation of sorted polygons under water. The predicted ratio of width to depth-of-sorting of 3.81 for sorted polygons is compared with the ratio of width to depth-of-sorting of 3.57 found from a linear regression analysis of 18 fie...
The various processes by which polymeric membranes are formed all involve the solidification of a... more The various processes by which polymeric membranes are formed all involve the solidification of an initially homogenous liquid phase. In order to validate a predictive model for these formation processed, it is necessary to make real-time noninvasive measurements of important process parameters. Although techniques have been described recently for the acquisition of mass and temperature data, no procedures have been reported for the measurement of instantaneous film thickness even though this is a particularly critical process variable. In response to this need, the authors have developed a novel laser interferometry technique which permits changes as small as 0.2 microns in film thickness to be measured. Utilizing this technique, real-time film thickness data for the cellulose acetate-acetone-water system have been obtained and compared with the dry-cast process model developed by the authors` group. It has been found that the experimental results and the model predictions are in good agreement. The theoretical basis for interference measurements, the experimental considerations in the utilization of a laser light source and the general applicability of the experimental technique also will be discussed.
ABSTRACT Better detection and control of fouling in liquid-separation processes is essential if m... more ABSTRACT Better detection and control of fouling in liquid-separation processes is essential if membranes are to find increased use in a variety of industrial applications. Ultrasonic time-domain reflectometry (UTDR) is an in situ, noninvasive real-time technique that has been successfully utilized to quantify fouling and cleaning of flat sheet membranes. This study describes the extension of UTDR for the measurement of fouling and cleaning in commercial membrane modules employing spiral-wound elements. Experiments were conducted using a high-pressure separation system incorporating a commercial spiral-wound module. Calcium sulfate was utilized as the primary scalant in concentrations ranging from 1.2-1.8 g/L; ferric hydroxide colloidal fouling also occurred during the tests. A multi-cycle protocol was adopted whereby each cycle consisted of three stages: pure water equilibration, fouling and cleaning. Permeation rate as well as acoustic amplitude and arrival time measurements were made at regular intervals during three complete cycles on two modules. A new signal analysis protocol was developed such that systematic changes in the entire acoustic spectrum as a function of module operation time could be represented in terms of shift factors. In addition, gravimetric, microscopic and spectroscopic measurements were made at the end of the experiments so that the extent of the membrane fouling and cleaning could independently assessed. Results indicate that the overall decline in permeation rate is reasonably correlated with change in both the amplitude and arrival time shift factors. Overall, this work confirms the effectiveness of the UTDR technique and provides a sound basis for continued development with an ultimate goal of providing on-line sensors for the timely detection of fouling and cleaning of commercial liquid-separation membrane systems.
ABSTRACT In this study an on-line feed fouling monitor (FFM) combined with a salt-tracer-response... more ABSTRACT In this study an on-line feed fouling monitor (FFM) combined with a salt-tracer-response technique (STRT) was used to predict reverse osmosis (RO) fouling under constant flux filtration. The FFM was used to capture foulant loads using a small ‘collection’ ultrafiltration (UF) membrane at the same crossflow hydrodynamics as in the RO experiments. A UF membrane was used in the FFM to decrease the monitoring time and improve the accuracy because it is more responsive to the fouling resistance than an RO membrane. Since the deposits captured by the FFM are potential RO foulants, the resulting information can be used to predict the transmembrane pressure (TMP) rise for the RO membrane. The STRT was used to measure the development of concentration polarization that is important in estimating the cake-enhanced osmotic pressure (CEOP) contribution. A model was developed that includes both the cake resistance and the CEOP effect due to cake formation and was used to predict RO fouling trends. Model foulants were humic acid and colloidal silica. The major focus was on organic fouling by humic acid (20 mg/l) in 2000 mg/l sodium chloride (NaCl) as the ionic background for the RO and FFM fouling experiments. The RO and FFM fouling experiments were conducted at different constant fluxes using the same feed solutions and at the same crossflow velocity (0.1 m/s). The results indicated that higher fluxes cause an increased fouling rate for both RO and the FFM for both types of solute. The CEOP effect, measured by the salt-tracer-response in the RO experiments, was also strongly enhanced by the flux. The model was validated by plotting the predicted RO transmembrane pressure (TMP) as a function of time for different fluxes based on the resistivity from the FFM and the CP obtained from the STRT. For both organic foulants and colloidal silica the results show that the combination of the FFM and salt-tracer-response STRT is a promising method to provide a good estimate of the RO fouling trends. It also underscores the contribution of CEOP to the increase in TMP during RO fouling of saline feeds.
ABSTRACT Ultrasonic Time Domain Reflectometry (UTDR) is used to monitor the deposition and physic... more ABSTRACT Ultrasonic Time Domain Reflectometry (UTDR) is used to monitor the deposition and physicochemical nature of colloidal silica fouling on a polyethersulfone ultrafiltration membrane under crossflow and constant flux conditions. The fouling can be characterized by four stages based on the rate of increase of the transmembrane pressure (TMP): (1) an initial rapid increasing rate due to concentration polarization; (2) a slow constant rate; (3) a nonconstant increasing rate associated with metastability of the colloidal silica; and (4) a constant rapid rate associated with Deleted ‘progressive compression of’. the foulant layer having a constant thickness. The destabilization of the colloidal particles in the foulant layer during stage 3 is associated with an increase in the UTDR peak amplitude. During stage 4 the foulant layer thickness reaches a plateau value that decreases with increasing crossflow velocity. The rheological behavior of the foulant layer is described by the Bingham plastic model for which the yield stress can be obtained from the UTDR data. When membrane cleaning is done by switching from a fouling to a non-fouling feed solution, marked decreases in both the foulant layer thickness and TMP are observed. However, a residual tightly bound foulant layer remains on the membrane whose thickness, determined by UTDR, corroborates well with off-line scanning electron microscopy analysis.
ABSTRACT In this paper RO colloidal fouling was predicted using an on-line Feed Fouling Monitor (... more ABSTRACT In this paper RO colloidal fouling was predicted using an on-line Feed Fouling Monitor (FFM) combined with a monitor based on ultrasonic time domain reflectometry (UTDR) under constant flux filtration. The FFM incorporated the relevant cross-flow hydrodynamics and detected the development of foulant load and resistance from a continuous sample passing over a small UF membrane. A UF membrane was used in the FFM, rather than a RO membrane, because it was more sensitive to fouling resistance and this decreased monitoring time and increased accuracy. The UTDR was coupled to a RO membrane cell to monitor the rate of cake thickness increase, which is the required information for the estimation of cake-enhanced osmotic pressure (CEOP). A model was developed to predict fouling in RO using data from the two monitors and combined both the cake resistance and the CEOP effect due to cake formation. RO and FFM fouling experiments were performed at different constant fluxes using the same feed solutions (200 mg/l SiO2, 2000 mg/l NaCl) and the same cross-flow velocity (0.1 m/s). The results showed that higher fluxes caused an increased fouling rate for both RO and the FFM. The foulant layer thickness measured by UTDR in the RO experiments increased faster at higher applied fluxes and the estimated CEOP effect was also strongly influenced by the flux. The results show that the model, combined with the FFM and UTDR measurements, can provide a good estimation of the RO fouling profile over a range of applied fluxes. This study also underscores the importance of CEOP in contributing to the increase in transmembrane pressure due to colloidal fouling in RO of saline feeds.
ABSTRACT Detecting biofouling non-invasively in real-time has been a long-standing challenge in m... more ABSTRACT Detecting biofouling non-invasively in real-time has been a long-standing challenge in membrane processes. Ultrasonic Time Domain Reflectometry (UTDR) offers the potential to address this challenge when used in a ‘canary cell’ that samples a slip stream from the feed to the membrane process. However, adapting UTDR to detect biofouling is problematic owing to the small difference in acoustic properties between a biofouling layer and water or a membrane. In this study UTDR has been adapted to detect biofouling by periodic dosing of colloidal silica as an ‘acoustic enhancer’. This novel UTDR technique was applied to detect biofouling on flat sheet polyethersulfone ultrafiltration (UF) and thin film composite polyamide reverse osmosis (RO) membranes in a canary cell flow configuration. The UTDR response correlates with the transmembrane pressure and off-line measurements of the biofilm thickness via confocal laser scanning microscopy, the bacterial count, and extra-polymeric cellular substances (EPS). The acoustic enhancer was shown to have no effect on the viability of the bacteria. Hence, UTDR can be used for the noninvasive real-time detection of biofouling in high pressure membrane processes such as RO by periodic dosing of an enhancer such as colloidal silica in a canary cell.
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Papers by William Krantz